using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

public partial class Window1 : Window
{
        public Window1()
        {
            InitializeComponent();

            Ellipse ellipseToInsert = new Ellipse();
            ellipseToInsert.Width = 150d;
            ellipseToInsert.Height = 100d;
            ellipseToInsert.Fill = new LinearGradientBrush(
                Color.FromArgb( 128 , Colors.Goldenrod.R, Colors.Goldenrod.G, Colors.Goldenrod.B),
                Color.FromArgb( 128 , Colors.HotPink.R, Colors.HotPink.G, Colors.HotPink.B), 20d);

            ellipseToInsert.SetValue(InkCanvas.TopProperty, inkcanvas.ActualHeight / 2d);
            ellipseToInsert.SetValue(InkCanvas.LeftProperty, inkcanvas.ActualWidth / 2d);
            inkcanvas.Children.Add(ellipseToInsert);
    }
}

<Window x:Class="Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="" MinHeight="500" MinWidth="500"
    >
  <DockPanel>
    <!-- Inking surface. -->
    <ScrollViewer DockPanel.Dock="Bottom" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
      <InkCanvas Name="inkcanvas"
                 EditingMode="Select" >
        <InkCanvas.Background>
          <RadialGradientBrush>
            <GradientStop Color="Snow" Offset="0" />
            <GradientStop Color="LightYellow" Offset="1" />
          </RadialGradientBrush>
        </InkCanvas.Background>
      </InkCanvas>
    </ScrollViewer>
  </DockPanel>
</Window>